Groundwater is water located under the Earth's surface. It is widely considered to be the most precious of all geologic resources. A large percentage of the Earth's population is dependent upon groundwater for drinking, irrigation, and/or industrial purposes. Fortunately, there is far more groundwater than water in all the world's rivers and lakes.
Groundwater occurs in the pores and fractures of rock and sediment. Although water contained in some materials is largely immobile, water contained in other materials is capable of migration in response to a pressure gradient and can be extracted by drilling. Such reservoirs of groundwater are often called aquifers. Water in most fresh-water aquifers has natural movement irrespective of any artificial withdrawals. This movement of groundwater has special significance in connection with groundwater contamination. When groundwater becomes contaminated at one place, the contamination tends to move to other places by transport down the hydraulic gradient. Accordingly, a contaminant source (e.g., a landfill or an accidental spill of a hazardous liquid) can be a threat to persons living great distances away who rely upon uncontaminated groundwater.
The science of geohydrology primarily addresses the physical occurrence of aquifers and the nature of groundwater dynamics--the complex flow systems in the subsurface. The principal controlling parameter in groundwater dynamics is the fluid pressure, specifically, the differences in fluid pressure in three dimensions. In accordance with Darcy's Law, groundwater migrates from high pressure to low pressure, quantitatively mitigated by: (1) the hydraulic gradient; (2) the hydraulic conductivity of the geologic medium; and (3) the cross-sectional area of the medium.
Mathematical analyses of groundwater conditions, specifically the physical parameters which control subsurface fluid dynamics, require the availability of precise information about fluid pressure. This information is generally obtained from a pumping test. In a pumping test, a central pumping well and one or more observation boreholes are drilled at points some distance from the pumping well. Each borehole, also known as a piezometer, is a relatively small diameter (about 5 to 30 cm) hole which extends down into the earth to the water-bearing formation. The borehole may be lined with a casing or partially unlined depending upon the geologic material. After carefully monitoring the water levels in the boreholes for a period sufficient to determine their long-term temporal behavior, the pumping well is turned on and a specified discharge is maintained for a predetermined period of time. While the duration of pumping varies with the particular physical situation encountered, the test normally extends for at least several hours and sometimes extends for a week or longer. From the instant pumping begins, the water level changes in the observation boreholes are carefully monitored. Water level changes are directly related to pressure changes because of fresh water' s uniform density. The drop in fluid pressure in the aquifer resulting from the pumping yields data which is then plotted as a function of distance from the pumping well and time after commencement of the pumping. Other things being equal, the greater the pressure drop observed in the borehole, the more resistance to flow in the water-bearing formation.
A variety of devices have been disclosed for measuring borehole groundwater levels during pumping tests, in particular, and for measuring fluid levels, in general. These devices generally fall within one of four categories: (1) tape measures; (2) mechanical floats; (3) immersion-conductivity sensors; and (4) ultrasonic devices. Tape measures are used by either extending them to the fluid level and observing the reading on the tape or by dipping them into the fluid, retracting them, and observing the fluid on the tape itself (much as is done with an automobile dipstick). The limitations of the use of tape measures in boreholes are obvious. First of all, the tape must be extended and then retracted for each reading unless the groundwater level is very close to the surface and, secondly, the tape method is incapable of accurately measuring fluid levels when the level is fluctuating rapidly, as groundwater levels usually do during the early, critical stages of a pumping test.
Floats move up and down with the fluid level and are generally connected mechanically to an indicator at the surface. A strip chart indicator moves with time to produce a recording of time and fluid level. Such a recording may be suitable for measuring relatively large changes in level over relatively long periods of time, but is of little utility in measuring small changes over short periods of time. Floats, like tape measures, are generally incapable of accurately measuring fluid levels which fluctuate rapidly. Floats are also prone to mechanical failure when left unattended.
Immersion-conductivity sensors are generally part of a probe which is lowered into a well. The sensor contains an electrical circuit which is triggered by the shunting effect of the conductive liquid when the probe reaches the fluid. An operator at the surface receives the signal and reads the footage indicator on the calibrated cable reel to determine fluid level. A variation of this type of device is disclosed in Fasching, U.S. Pat. No. 4,523,465, issued June 18, 1985, where the probe sends an acoustic signal, rather than an electrical signal, up the well when the fluid is contacted. The use of immersion-conductivity sensors requires an operator to raise or lower the probe for each reading and is a relatively slow and cumbersome process.
Ultrasonic devices of various types have been used for many years to determine fluid levels in wells. The devices generally operate by sending an ultrasonic signal from ground level down the full depth of the well and then recording the signal reflected upward from the fluid surface. If the speed of sound were constant, distance could be determined by merely measuring the elapsed time from the transmission to the receipt of the reflected signal. However, the speed of sound is a function of temperature, pressure, and the composition of the medium through which it travels. The variation in speed of sound, coupled with the large distances involved (fluid levels in wells are often 100 or more meters below the surface), requires some type of calibration to be used.
For example, Ritzmann, U.S. Pat. No. 2,232,476, issued Feb. 18, 1941, discloses the use of an ultrasonic device for measuring fluid level in a well having a casing and tubing. The distance to the fluid is calibrated by noting the number of reflections from the tubing collars, which generally occur about every 30 feet. Godbey, U.S. Pat. No. 4,318,298, issued Mar. 9, 1982, discloses an ultrasonic device in which the acoustic pulse source is automatically actuated at predetermined times, and the depth to the liquid level and the time are automatically recorded to produce a record of time and liquid depth which can extend over a period of several days. The Godbey device is calibrated by using the known distances between tubing collars or by using the known depth of the fluid at a given point in time. The device allegedly enables variations in the fluid depth of only a few feet to be observed without any interpretative errors. However, the Ritzmann and Godbey devices require prior knowledge of the well's construction and such information is often unavailable.
Ultrasonic devices have also been disclosed for measuring fluid levels in tanks and other large vessels. Some of these devices employ a fixed hollow cylinder extending from above to below the surface of the fluid. In some devices, the cylinder is used as a wave guide (the sonic pulses are transmitted through the cylinder itself) while in other devices the cylinder is used to contain the sonic pulses which move through the fluid within the cylinder. The devices employ different calibration techniques. For example, Turner, U.S. Pat. No. 2,713,263, issued July 19, 1955, employs radial slots on the outside of the cylinder and Tomioka, U.S. Pat. No. 3,394,589, issued July 30, 1968, employs reflector elements inserted at given distances in the cylinder. Willis, U.S. Pat. No. 3,834,233, issued Sept. 10, 1984, employs a second receiver which is positioned a fixed distance from the transmitter-receiver. Shuler, U.S. Pat. No. 4,090,407, issued May 23, 1978, and Austin, U.S. Pat. No. 4,170,765, issued Oct. 9, 1979, disclose cylinders having one calibration target a fixed distance from the ultrasonic transmitter-receiver.
In contrast to the above devices which measure the level of groundwater (from which pressure is then derived), there are other devices which apparently measure pressure directly. Such a device is disclosed in McKee, U.S. Pat. No. 4,461,172, issued July 24, 1984, as being useful in generating data for a drawdown test.
The importance of groundwater-level-measuring devices (and the data they obtain) to geohydrologists cannot be overstated. The precision and frequency with which changes in groundwater pressure are made determines the accuracy and value of all subsequent calculations based upon the pumping test. Accordingly, the persons conducting pumping tests and those using the test results would like to be able to measure with extreme accuracy very small changes in groundwater pressure over very short periods of time. For example, an apparatus which could accurately measure groundwater level changes of only about 2 mm (which corresponds to a change in fluid pressure in the aquifer of about 0.0002 atm), could make and record such measurements about every second, and could do so without site specific calibrations would be a tremendous advance over the devices and methods currently in use.